1. Field of the Invention
The present invention relates to an improved optical disk substrate and an optical disk using the same.
2. Description of the Prior Art
To detachably attach an optical disk to a drive unit a variety of magnetic type clamps have been widely used.
Japanese Patent Application Kokai No. 1-253886 shows a clamping structure using a clamp plate of a magnetic metal to be fitted in the center hole of an optical disk substrate as shown in FIG. 1 of the publication, and another clamping structure using a resin piece having a magnetic metal piece impregnated, said resin piece being fixed to an optical disk substrate, thereby permitting the optical disk to be attracted and fastened by a magnet piece of the underlying drive unit as shown in FIG. 2 of the publication. As for the former clamping structure clamp plates must be made by press forming. As for the latter clamping structure resin pieces each having a magnetic piece impregnated must be prepared. Thus, these prior art clamping structures require extra work and accordingly extra cost.
FIG. 3 shows still another clamping structure using a flat clamp plate 4 attached to one major surface of the disk substrate. Advantageously this clamping structure can be produced at a reduced cost. The optical disk can be attached to the drive unit in correct position by inserting the spindle of the drive unit in the center hole 3 of the disk substrate 2. Preferably the tip of the spindle is tapered or chamfered to facilitate insertion of the spindle of the drive unit in the center hole of the disk substrate. This, however, causes substantial reduction of the contact area in which the spindle and the inner wall of the center hole come to contact because there remains an annular gap between the tapered tip of the spindle and the inner wall of the center hole of the disk substrate. In an attempt to compensate for such reduced contact area an annular projection of substantial longitudinal extension is integrally connected to the inner circumferential area of the disk substrate 2 as indicated at 23, accordingly extending the center hole 3 longitudinally. Some drive units require extension of an increased distance from the opening end of the center hole 3 to the clamp plate 4, compared with the thickness of the disk substrate, and in this case it is necessary that an annular projection be integrally connected to the inner circumferential area of the disk substrate.
Such hub-attached disk substrates can be injection molded. The inventor found that such injection-molded articles each having a 0.5 mm or more high hub projection cause appearance of increased birefringence of light in the disk substrate, particularly at radially inner portion of the information-bearing section of the disk substrate. Usually a fine beam of light is thrown to the rear side of the optical disk to reproduce the information recorded in the disk. The refraction of light in passing through the disk, however, will be a cause for reducing the quality of reproduction from the optical disk.